1. Field of the Invention
The present invention relates to the field of cellular telephone networks. Specifically, the present invention is directed at conserving battery power for a cellular telephone.
2. Description of Related Art
In the prior art GSM System, a service area, which can be a metropolitan city, is subdivided into cells. For every group of cells is a base station which communicates with one or more mobile stations (i.e., one or more mobile telephones, each referred to as a "mobile"). The base stations in turn are connected to a base station controller. In addition, a mobile switching center (MSC) is used to communicate with all the base station controllers in the GSM system and to keep track of the latest position of each of the mobiles. Thus, the MSC knows in which cell each mobile is currently located.
The GSM system needs to know the mobile's latest position because the MSC uses that information to "page" the mobile--i.e., the location information is used to notify the mobile of an incoming call. The MSC sends the paging message only to the cell in which the mobile is located so as to avoid the resource wasteful technique of sending a paging message over all cells. For example, when the mobile moves from a first cell to a second cell in idle mode, the mobile signals the MSC to inform the GSM system that the mobile has changed cells. Subsequently, when the GSM system wants to signal the mobile, the GSM system transmits the paging message only in the cell where the mobile is currently located.
Thus, a major concern which exists in the GSM system is updating the location of each mobile--i.e., the GSM system has to be kept aware of in which cell each mobile is currently located. Moreover, the GSM system has to be aware of the mobile's location when the user is constantly moving, regardless of whether the user is on the phone (i.e., the mobile is idle).
To accomplish this in the GSM system, each cell constantly broadcasts its identity and other relevant parameters on a beacon signal using a beacon frequency. Each cell has a different beacon frequency from its neighbor. A mobile detects movement from one cell to the next by periodically detecting and comparing the signal strength of the beacon signal of the current cell (i.e., the cell in which the mobile is currently located), to the signal strengths of the beacon signals of the surrounding cells. During the detection process, the mobile receives information that is needed to identify each cell on a broadcast control channel (BCCH). In addition, for the present cell, the mobile has to monitor and decode a paging channel (PCH) to detect the presence of incoming calls. The BCCH and PCH are contained on this beacon signal.
The monitoring and comparison process is performed every 30 seconds. If the signal strength from one of the other cells becomes more powerful than the present cell from a radio power perspective, the mobile will switch to that other cell and notify the GSM system.
FIG. 1 is a diagram of the prior art GSM system having a set of cells C1, C2, and C3, each having base station transceiver BST1, BST2, and BST3, respectively, located therein. Base station transceivers BST1, BST2, and BST3 communicate with base station controller BSC1 of a set of BSCs BSC1 through BSCn. Set of BSCs are connected to mobile switching center MSC1. A mobile MS1, located in cell C3, communicates with MSC1 through the use of base station BST3 and base station controller BSC1.
As described above, to allow the GSM system to locate mobile MS1, mobile MS1 is responsible for notifying mobile switching center MSC1 every time mobile MS1 moves from one cell to another. This notification is termed a "registration" and occurs only if mobile MS1 is in idle mode and mobile MS1 happens to change cells. The traffic that results from mobile MS1 notifying mobile switching center MSC1 of the present cell location of mobile MS1 is called registration traffic. For example, as mobile MS1 moves out of the present cell, cell C3, and the signal strength of the beacon signal from one of the surrounding cell (e.g., either cell C1 or cell C2) becomes more powerful than the one from the present cell, mobile MS1 will decide to switch over to that cell. After mobile MS1 switches to the new cell, mobile MS1 will send a registration to MSC1.
FIG. 2 is a flow diagram of the prior art monitoring and decoding process illustrating the typical operation of a mobile in the GSM system. The mobile has been powered-up and placed in idle mode (i.e. the user has turned on the mobile but has not initiated or received a call with the mobile).
In block 102, the mobile detects and synchronizes the mobile's internal clock to the transmission of the present cell. In FIG. 1, for example, mobile MS1 synchronizes itself with cell C3. Thus, the mobile selects cell C3 (depending on signal strength or other reasons), synchronize to it and makes sure that it has successfully decoded the BCCH and other info that comes on the beacon signal of cell C3. After synchronization has occurred, operation will continue with block 104.
In block 104, the mobile decodes the BCCH portion of the transmission of the present cell.
In block 106, the mobile measures the signal strength of the beacon signal of the present cell. In FIG. 1, for example, mobile MS1 measures the signal strength of the beacon signal of cell C3. The signal strength is stored for comparison of the signal strengths of the beacon signals for the surrounding cells.
In block 108, the mobile decodes the PCH for paging messages which will indicate to the mobile that there is an incoming call designated for the user.
In block 110, if the decoded PCH from block 108 contains a paging message, then operation will continue with block 112. Otherwise, operation will continue with block 114.
In block 112, the mobile has detected a paging message during the decoding of the PCH in block 108. Thus, the mobile enters into a dedicated mode to receive the call.
In block 114, if the mobile has not found a paging message during the decoding of the PCH in block 108, then the mobile will synchronize to a beacon signal of a surrounding cell. For example, referring to FIG. 1, the mobile synchronizes the monitoring and timing circuits of the mobile to the beacon signal of cell C2, which is a cell adjacent to the present cell, cell C3.
In block 116, the mobile decodes the BCCH of the surrounding cell. Operation will then continue with block 118.
In block 118, the mobile measures the signal strength of the beacon signal of the surrounding cell and store the value into a table containing a list of cells and the signal strengths of the beacon signals of each cell.
In block 120, the mobile re-orders the list of signal strengths of the camped cell and surrounding cells in an increasing order of beacon signal signal strengths.
If the beacon signal signal strength of the surrounding cell is stronger than the beacon signal signal strength of the present cell, indicating that the mobile has moved into the surrounding cell, then the mobile will notify the GSM system that the surrounding cell is now the new present cell so that the GSM system will send all paging messages only to the new present cell.
If the signal strength of the beacon signal of the present cell is still the strongest, then the mobile will know that it is still in the present cell, and the monitoring and processing cycle will end until the time of the next cycle.
It is to be noted that operations contained in block 114 to block 120 is repeated until the signal strength of all surrounding cells have been measured and inserted into the cell table. Thus, for example, in FIG. 1, the BCCH and beacon signal signal strength of cell C1 is captured and measured, respectively, before the mobile proceeds with block 120.
Thus, in the GSM system, the mobile has to monitor the signal strength of the beacon signals of all surrounding cells every 30 seconds in addition to monitoring the signal strength of the beacon signal of the present cell to be able to determine whether or not it is moving. However, every time the mobile monitors the surrounding cells, the mobile has to expend power as the circuits which are used to perform the monitoring have to be supplied power while monitor the additional surrounding cells. In addition, the processing logic such as that used to determine the strongest beacon signal also expends power to perform the comparison functions.
For example, in FIG. 1, mobile MS1 not only has to expend power to monitor the present cell, cell 3, but also has to expend power monitoring the surrounding cells, cell C2 and cell C1. Moreover, MS1 would also have to expend power to re-order the signal strengths of the beacon signals of the present and surrounding cells. This reordering occurs so that when the mobile leaves the present cell, the mobile knows which surrounding cell has the highest signal strength and use that surrounding cell as the new present cell.
This periodic monitoring and comparing is wasteful in cases when a user is be fairly stationary and not be moving from cell to cell. For example, the user can go inside a building and still stay in the same cell. The power consumption is especially wasteful when there many surrounding cells, such as in a metropolitan area. Thus, if the user will be in a general area, performing this periodic monitoring and comparing of surrounding cell beacon signal strengths is unnecessary and it would be desirable to eliminate this period monitoring and comparing.